Tailings facilities from the recovery of minerals have historically and still continue to create a legacy for the industry, and for the communities which host the mining and mineral processing operations. The tailings are a mix of silt (<75 micron), fine sand (75-150 micron), and coarse sand (>150 micron) from which most of the valuable component of the ore has been recovered using beneficiation techniques such as flotation. The high silt content causes the tailings to have a low hydraulic conductivity, which although they may vary widely, are typically around 10-5 cm/sec which means the tailings do not drain freely.
Fine grinding of the ore (e.g. copper, gold, zinc/lead, nickel, platinum group metals, etc.) is required to liberate the values from the containing gangue, to selectively float the values in a froth flotation cell. For copper, this size range is typically a p80 of between 100 and 200 micron. Consequently, all the gangue materials associated with the valuable mineral are comminuted to a similar size. The resultant tailings are usually stored as a thickened slurry or paste in a purpose built tailings storage facility (TSF) constructed at a significant capital cost.
With reference to FIG. 1, in a conventional fine flotation circuit, run of mine (ROM) ore 10 from blasting and crushing 12 is ground 14 and classified 16, typically in a closed circuit, returning the oversize material 18 from classification for further grinding, to ultimately produce the required size for flotation. Classified ore is subjected to fine flotation 20, to obtain concentrate 22. Tailings 24 from the fine flotation 20 are thickened in a thickener 26 and stored in a TSF 28.
Various techniques have been proposed, to avoid or minimize the amount of tailings to be stored in a TSF.
In some tailings disposal systems, the coarsest fraction of the tailings, containing mainly a fine sand, is separated by a cyclone, and a proportion of the tailings (typically 5-30% of the total ore depending on the required grind size for flotation) is separately stacked and drained. This modest reduction in quantity of tailings is limited by the need to maintain a free draining sand for disposal. For example, where this sand is used as in a load bearing dam wall, the sand typically requires less than 15% silt content. The residual tailings still require storage in a large purpose built dam.
Increasingly, in areas where the location of the TSF is particularly problematic and water is scarce, the reject slurry is filtered and deposited as a solid sludge containing around 15-20% water. The filtration is costly, due to a slow filtration rate resulting from the low hydraulic conductivity of the tailings (typically containing around 50% silt). The filtered residue has little structural integrity, and remains subject to mud formation and erosion during rain periods and excessive dusting if disturbed.
Various proposals have also been made for blending some fraction of the tailings together with waste rock from the mining process. The fine tailings reduces permeability of the waste rock and hence its propensity for acid generation. However this technique does not enable recovery of significant water from the tailings, and has not been widely adopted by the industry.
The reasons include the mismatch in quantities of tailings and waste rock at different periods of the mine life, and the difficulty in effective distribution of the fine sludge through the waste rock in appropriate proportions.
In recent years, an alternative to conventional fine flotation has been proposed. (G. J. Jamieson, Aus. I. M. M. G.D. Delprat Distinguished Lecture Series, Sydney, 2013) This modification as illustrated below, is termed coarse flotation, and has been promoted as a method of reducing the energy costs associated with fine grinding of ores.
With reference to FIG. 2, in coarse flotation, run of mine (ROM) ore 30 from blasting and crushing 32 is ground 34 to a coarser size than that for conventional fine flotation, such that much of the valuable minerals are partially exposed, but not fully liberated. Typically the ideal particle distribution of the feed to a coarse flotation is sized between 150 micron and 1 mm. The upper size is limited by either the design of the flotation equipment or the particle size at which a significant proportions of the valuable component of the ore grains are no longer exposed on the surface of predominantly gangue particles. For copper or gold ores, this upper exposure limit occurs typically at a particle size of around 300-700 micron.
The lower size limit for coarse flotation is determined by the relatively inefficient separation of fines in the coarse flotation equipment. For example, with the commercially available Eriez Hydrofloat cell, the lower size limit is typically around 150 micron, as much of the smaller particles will simply be entrained in the teeter water.
The ground ore is classified in a first classifier 36, typically in a closed circuit, returning the oversize material 38 from classification for further grinding. The classified ore is further classified in a second classifier 38. A fraction of the ore in the selected operational size window for coarse flotation (say 150 micron to 700 micron.) is separated from the remainder of the finer and coarser ROM ore, and is floated in a specially designed coarse flotation cell 40, to produce an intermediate concentrate. The oversize material is recycled for further comminution and the finer material from the pre-classification (typically fine sand and silt at <150 micron, and accounting for 50% or more of the total ROM) is directed to conventional fine flotation 42. The intermediate concentrate produced in coarse flotation, typically 5-20% of the original feed, is reground 44 to a size where the valuable fraction in the ore is liberated and suited to producing a saleable concentrate. It is directed to further beneficiation through conventional flotation 42, along with the fines fraction from classification. This conventional or fines flotation process yields a final concentrate product 45 and a fine tailings residue. Tailings arising from the both the coarse and fine flotation are then combined, thickened 46 and deposited in a TSF 48.
One example of such a coarse flotation cell is the Hydrofloat cell, manufactured by Eriez (U.S. Pat. No. 6,425,485 B1, 2002). The potential for application of this cell for treating copper, gold, and other sulphide ores is described in numerous papers and conference proceedings (such as J. Concha, E. Wasmund http://docplayer.es/10992550-Flotacion-de-finos-y-gruesos-aplicada-a-la-recuperacion-de-minerales-de-cobre.html.) There are also other coarse flotation cell designs, and other related methods have been proposed for separating partially exposed coarse particles from gangue, by selective attachment of a collecting agent and flotation. For simplicity, all these alternative separation technologies, will all be termed coarse flotation.
It is an object of this invention to provide an integrated processing system utilising coarse particle flotation to eliminate or reduce the requirement for a tailings storage facility (TSF).